1. Field of the Invention
The present invention relates to a zoom lens and a focusing method used for a photographing lens for a 35 mm lens shutter camera for example and a photographic lens for a video camera.
2. Description of the Related Art
Various kinds of zoom lenses are generally known. Japanese Patent Application Laying Open (KOKAI) No. 63-25613 discloses three groups of lenses having negative, positive and negative focal distances as a recent zoom lens. This zoom lens is constructed to perform a zooming operation without any singular point from a wide angle end to a telescopic end.
The inventor of this application also proposed a compact zoom lens constructed by three groups of lenses having negative, positive and negative focal distances as mentioned above in Japanese patent application No. 63-215967.
The zoom lens disclosed in the above laid-open publication has the problems that it is difficult to increase a zoom ratio since the first lens, group is fixed. In the above compact zoom lens, back focus is minimized at the wide angle end and the distances between the respective lens groups are minimized at the telescopic end so that it is difficult to reduce the focal distance at the wide angle end while the focal distance at the telescopic end is held.